The objective of this study is to determine the mechanism of nelfinavir-associated diarrhea in HIV-infected individuals. Diarrhea was defined as 300 grams/24 hours. Prior to hospitalization, patients were screened with a 24-hour stool collection. Stool specimens were analyzed for ova and parasite, culture, and C. difficile toxin. Subjects were placed in a lactose free diet 5 days prior to coming in to the hospital and during the inpatient phase (in order to eliminate other potential causes of diarrhea). Eligible patients were hospitalized for a 48-hour stool collection.